1. Field of the Invention
The present invention relates to a sealing structure for a piston which axially reciprocates in a cylinder, and further to a piston pump and a brake hydraulic pressure control device each incorporating the sealing structure.
2. Discussion of the Related Art
As sealing structures of this kind, there has been known one wherein a seal member made of a synthetic resin material which is pressed radially outward by an O-ring is provided in an annular groove formed on an outer circumferential surface of a shaft member (refer to Japanese Patent No. 4081800 B2 for example). In this structure, an urging force of the O-ring serves to perform a sealing function between an outer circumferential surface of the seal member and an internal surface of the cylinder in which the shaft member is fitted.
In the known sealing structure, peak regions in surface pressure (i.e., contact force or strength per area) against the internal surface of the cylinder are formed at two axially spaced places on the outer circumferential surface of the seal member. Thus, in the Japanese patent, it is mentioned that the posture of the seal member relative to the cylinder can be stabled, so that it is possible for the plurality of sealing portions (i.e., the aforementioned peak regions) to enhance the sealing performance between the shaft member and the cylinder.
The inventors of the present application found out the following problem in the sealing structure disclosed in the Japanese patent wherein a seal member performs a sealing function at axial end portions thereof. That is, the sealing performance becomes insufficient in a certain situation. When the shaft member axially reciprocates in the cylinder, it occurs that the posture of the seal member changes repeatedly in dependence on the moving direction of the shaft member. In this case, because the peak regions of the sealing surface pressure (hereafter referred to as seal portions) are formed at plural places spaced axially on the sliding surface, the surface pressures at the respective seal portions change alternately in dependence on the change in the posture of the seal member. Thus, the thicknesses of oil films at the respective seal portions are changed by the changes in the surface pressures, and this may result in that the fluid in a closed chamber is gradually drawn out to the exterior (this is called a pumping phenomenon).
More in detail, when the shaft member first moves in a positive-going direction, the change in the posture of the seal member occurs which causes the fluid in the closed chamber to leak from one seal portion being lowered in the surface pressure, and the leaked fluid enters a space between both of the seal portions. Then, when the posture of the seal member changes in the reverse direction in the return movement of the shaft member, the fluid which entered the space between both of the seal portions leaks this time from the other seal portion lowed in the surface pressured to be discharged outside.
In particular, in a sealing structure wherein a seal member pressed by an O-ring radially inward is provided in an annular groove formed in a cylinder and wherein flanges for preventing the coming-off of the O-ring are formed at axial end portions on the outer circumferential surface of the seal member, the O-ring presses either of the flanges in the axial reciprocation movements of the shaft member, and this makes the posture of the seal member further unstable. Further, where a high hydraulic pressure is being generated in a closed chamber as is the case of a hydraulic pump, the high hydraulic pressure in the closed chamber contributes to the change in the posture of the seal member, so that the aforementioned pumping phenomenon becomes remarkable.
By the way, in recent years, a vehicle brake hydraulic pressure control device is being used for controls in a wide range which include not only controls such as a so-called antiskid control, vehicle stability control (side slip control) and the like, but also controls such as brake force assist control or automatic brake control in the case of travelling to follow a vehicle ahead. With this, the vehicle brake hydraulic pressure control device is remarkably increasing in the number of operation times and the operation period of time. Where the number of operation times and the operation period of time increase, the chances for a piston pump to operate in the device also increase, so that there arises an apprehension that the chances for brake fluid to leak outside of the pump form a sliding portion (sealing portion) between a piston and a cylinder receiving the piston therein also increase. Thus, it becomes a problem to reduce the leak quantity of brake fluid as little as possible as a measure against such an apprehension.
Under these circumstances, a prior art piston pump wherein the sealing between a piston and a cylinder is carried out by the use of only a seal member made of a rubber material (e.g., one that is configured by removing a seal ring 11 made of a synthetic resin material from a piston pump shown in FIG. 2 and that uses an O-ring 12 as seal member) is accompanied by an apprehension that the wear and the scraping of the rubber part forming a sliding portion increase to the extent that is unable to neglect, with increases in the number of operation times and the operation period of time in the brake hydraulic pressure control device, and this gives rise to a problem in reducing the leak of brake fluid.
There has been known another piston pump in which a coating which smoothens the surfaces of a piston and a cylinder is carried out in order to prevent the wear and scraping of the rubber part (e.g., US2008/0069709 A1 equivalent of DE102004010498 A1). In this device, an apprehension arises in that the exfoliation of a coated layer takes place with increases in the number of operation times and the operation period of time.